C++ constexpr parlor tricks: How can I obtain the length of a string at compile time?

Raymond Chen

November 14th, 20227 3

Say you want to obtain, at compile time, the length of a compile-time string constant. The problem is that the strlen function is not constexpr.

#include <cstring>

template<std::size_t> void tryme();

void test()
{
    // error; expression did not evaluate to a constant
    char buffer[strlen("hello") + 1];

    // error: invalid template argument
    tryme<strlen("hello")>();

    switch (0)
    {
    // error: case expression is not constant
    case strlen("hello"): break;
    }
}

Note that gcc and clang support variable-length arrays as a nonstandard extension, so you may get away with the buffer declaration unless you turn off that extension. In fact, gcc goes further and accepts all three statements!

How can you get all three of the above to work in standard-conforming code? One idea is to write your own constexpr_strlen. But it turns out that somebody already wrote it for you, although it has a rather awkward name: std::char_traits<T>::length().

#include <string>

constexpr std::size_t constexpr_strlen(const char* s)
{
    return std::char_traits<char>::length(s);
    // or
    return std::string::traits_type::length(s);
}

constexpr std::size_t constexpr_wcslen(const wchar_t* s)
{
    return std::char_traits<wchar_t>::length(s);
    // or
    return std::wstring::traits_type::length(s);
}
Raymond Chen